"NMEA0183 is a standard describing both the electrical characteristics
and the software protocol used for interfacing electronic marine
navigational devices. Version 1.x, the original standard, is an
unbalanced signal, similar to RS232, but with higher current handling
capabilites and optical isolation on the receive line. The newer
standard, version 2.0 and above, uses an RS422 transmitter combined with
an optically isolated receiver"